Small amounts of rotor shaft unbalance tend to lead to radial vibration and eccentricity of the shaft and its bearings, that must be damped by means of devices for compressing a film of oil (squeeze film dampers) associated with the bearings for supporting and guiding the shaft.
Each squeeze film damper comprises a housing formed in a support case of the corresponding bearing in which the outer ring of the bearing is mounted with a small amount of radial clearance. An annular space defined around the ring in said housing is filled with oil and is closed axially by annular sealing elements that are free to rotate in annular grooves of the outer ring of the bearing and that co-operate in sealed manner with an inner cylindrical surface of the case.
Oil inlet orifices are formed in the case and open out into the above-mentioned annular space, and oil outlet orifices are formed in the annular sealing elements and open out to the outside of said annular space so as to enable oil to be caused to flow continuously through the annular space and be cooled outside said space so as to remove the heat energy dissipated by the friction that results from the oil film being compressed by the outer ring of the bearing during its orbital movement in the above-mentioned housing. Without cooling, the temperature rise of the oil in the annular space would lead to a significant drop in its viscosity and thus in its ability to damp the orbital movements of the outer ring of the shaft bearing.
For the shaft of an airplane turboprop or turbojet, the annular sealing elements are generally made of segments that withstand high temperatures, said segments being split rings of elastically-deformable metal, and conventionally they are of rectangular section. The splitting slots in such segments may be cross-cut or they may be cut with an overlap, and they form oil outlet orifices. In order to ensure an oil leakage rate that is sufficient, it is preferable to use cross-cut segments rather than overlap-cut segments, and to make a step in the outer periphery of each segment in register with its slot so as to calibrate the oil leakage flow rate.
Oil leakage is thus localized at a specific point around the circumference of the segment. Nevertheless, the rate at which oil leaks through the slot is observed to vary strongly as a function of the angular position of the outer ring of the bearing relative to the slot. Analysis of the behavior of the damper has shown that the outer ring of the bearing is subjected both to a stationary (non-rotating) hydrostatic force and also to a rotating hydrodynamic force, with the hydrostatic force being of the same order of magnitude as the hydrodynamic force, and being due to the oil leaking through the slot in the segment.
The stationary hydrostatic force imparts behavior on the compressed film of oil that is not axially symmetrical and that is uncontrolled since the segments are free to rotate in the grooves of the outer ring of the bearing, which means they can be rotated in random manner by friction, thereby causing the angular positions of the slots in the segments to vary in uncontrolled manner relative to one another and relative to the outer ring of the bearing.
As a result, the behavior of the oil film is not controlled and the radial vibrations and eccentricity of the shafts cannot be damped in completely effective manner.